In addition to using space in the i/o address space, this card also uses 4 Kilobytes
address space in the main memory address space. It may be configured
in increments of 0x2000 within the space of 0xc0000 to 0xde000. Usually, VGA
cards occupy space from 0xc0000 to 0xc7fff, so you have to use something from
0xc8000 upward. 0xd0000 or 0xd8000 are sometimes free, so lets assume for now
no other card in your machine occupies the 4Kb space from 0xd0000 and we use
this value to configure our card.